A glass preform for use in the fabrication of an optical fiber comprises a core and a cladding surrounding the core. The core must have a higher refractive index than the cladding so as to allow easy propagation of light therethrough.
In order to increase the refractive index of the core higher than that of silica, additives such as TiO.sub.2, GeO.sub.2 and Al.sub.2 O.sub.3 are usually added to the core material. Among them, GeO.sub.2 is most commonly used (cf. Japanese Patent Kokai Publication (unexamined) Nos. 127744/1976 and 46742/1078). In a usual optical fiber, pure quartz glass is often used to form the cladding. In this case, pure quartz glass has a refractive index of 1.4585 and .DELTA.n =0.
Referring to FIGS. 1A and 1B, there are shown diagrams illustrating distributions of the refractive index of two types of optical fibers. In these figures, the regions A and B indicate the core and cladding, respectively. The difference in refractive index between the core and cladding is usually indicated in terms of a relative refractive index difference (in percent). That is, assuming that the refractive indices of the core and cladding are n.sub.1 and n.sub.2, respectively, the relative refractive index difference .DELTA.n % is represented by the following equation: ##EQU1##
FIG. 1A shows the general distribution of refractive index of a single mode optical fiber. In this case, .DELTA.n is usually 0.3 to 0.5%. FIG. 1B shows the general distribution of refractive index of a multi-mode optical fiber. For an optical fiber for ordinary communication purposes, n is usually about 1%, and for large aperture optical fibers used in computer ring communication applications, n.sub.12 is usually about 2 to 4%.
Oxide additives such as GeO.sub.2 added to increase refractive index of the core cause light scattering (Rayleigh scattering) because of their inherent characteristics. As the amount of the additive added is increased, the degree of light scattering (Rayleigh scattering) due to the additive increases. This is not desirable for light transmission.
If the additive is added in a large amount, bubbles and/or a crystal phase are formed in the glass preform. In the case of GeO.sub.2, for example, GeO gas easily forms, thereby producing bubbles. In the case of Al.sub.2 O.sub.3, clusters of Al.sub.2 O.sub.3 crystals easily forms. This is not desirable for light transmission characteristics and also for the strength of the optical fiber. Furthermore, the coefficient of thermal expansion of glass increases, which makes the glass preform fragile. Therefore, also from the viewpoint of light propagation and glass strength, it is preferred to reduce the amount of the additive added to the core.
For this reason, it is proposed to increase the refractive index difference between the core and cladding by lowering the refractive index of the cladding. For example, additives which lower the refractive index, such as B.sub.2 O.sub.3, fluorine or a combination thereof, can be added to the cladding (cf. Japanese Patent Kokai Publication (unexamined) No. 111259/1982). However, B.sub.2 O.sub.3, has the disadvantage that the coefficient of thermal expansion of the resulting cladding greatly changes with the concentration of B.sub.2 O.sub.3 and the refractive index changes upon heating. Furthermore, with regards to light transmission, the cladding has an absorption loss due to B.sub.2 O.sub.3 in a longer wavelength region. Thus, it is preferred to use fluorine as a refractive index-lowering agent.
The addition of fluorine to quartz glass produces optical fibers with various refractive index distributions, and with the proper choice of structure, an optical fiber of low dispersion over a wide wavelength region can be obtained.
An advantage obtained by using fluorine as an additive is that, since the refractive index of the cladding can be made lower than that of pure quartz, pure quartz or quartz glass with a small amount of additive added thereto can be used in the fabrication of the core.
In addition, GeO.sub.2 as an additive to increase the refractive index and fluorine as an additive to decrease the refractive index may be simultaneously added to the core and the cladding, respectively.
An optical fiber having a distribution of refractive index shown in FIG. 2 has been proposed (cf. A. D. Pearson, et al., Fabrication and Properties of Single Mode Optical Fiber Exhibiting Low Dispersion, Low Loss, and Tight Mode Confinement Simultaneously, The Bell System Technical L., Vol. 61, No. 2 (1982) 262) and the new optical fiber comprises a core made of GeO.sub.2 --SiO.sub.2 glass and a cladding made of SiO.sub.2 --F--P.sub.2 O.sub.5 produced by MCVD (Modified chemical vaporphase deposition) method.
However, the structural instability of the optical fiber comprising the core made of GeO.sub.2 --SiO.sub.2 glass was revealed by UV absorption peak and coloring by radiation observed therein. Needless to say, the structural deficiency of the glass structure adversely affects light transmission characteristics even in the near infrared region in which the optical fiber is used.